Many low quality fuels, such as blast furnace gas, and excessively diluted fuel gases such as methane-containing coal mine ventilation air, are flared or vented and thus not beneficially utilized due primarily to problems associated with the very high cost of established techniques and/or the non-availability of suitable techniques and low cost equipment enabling the fuel's commercial utilization. Very large volumes of these such fuel gases must be compressed to be utilised, and the mass flow rates through the compression and expansion stages of the gas turbine, and the associated combustion problems, are such that standard turbine designs cannot be used.
A further problem with the use of gas turbines to utilize diluted fuel gases is that substantially all of the gas must be compressed to above the combustion pressure of the turbine to be fed as fuel to the turbine, and this compression is costly both in equipment and power requirements.
In gas-prone underground coal mines, methane exaction may remove part of the total methane present in coal seams affected by underground mining as a methane fuel gas stream contaminated with air and other contaminants, such as carbon dioxide. The ratio of methane extracted by this technique is generally less than 50% of the total quantity of methane emitted during mining, with the greater part being emitted in the mine ventilation air. The methane content in ventilation air in these mines typically is between 0.8 and 1.0%. It has been proposed to utilize this otherwise wasted gas by ingestion in gas turbines as all or part of the air feed to the turbine, although it has been found that modem high efficiency and cost-effective gas turbines are generally not suited to this duty, as the required overall ratio of methane fuel to ingested air ratio for such turbines is generally in excess of 2% and normally closer to 3% by volume, hence even if the ingested methane in the methane-contaminated air could be burnt, and even with 1.0% by volume of methane in the air, only approximately one third of the turbine's fuel requirements would be provided. Such a turbine could not make effective use of the gas flows from such mines even if the combustion problems could be solved and mine drainage gas used as supplementary fuel.
With such dilute fuel gas steams, a further problem exists in that modern high efficiency turbines typically use in excess of 15% of the total air flow for cooling and purging purposes. Consequently a significant part of the methane in the ingested air by-passes the combustion stage, hence any fuel in that air can not be utilized and thus is wasted. In addition, it is known tat methane by-passing the combustion stage and being added to the combustion gas flow at intermediate temperatures can form active radicals such as methyl-hydroxyl radicals which promote the conversion of nitric oxides formed in the combustion stage to toxic and visible nitrogen dioxide.
Some low purity gases such as those from sewage treatment, "landfill" operations, coal seams and the like may contain high concentrations of carbon dioxide, up to and possibly in excess of 70% by volume. Such gases have the problem of being very difficult if not impossible to combust in conventional gas turbines and related combustion systems.
A paper entitled "The Elimination of Coal Mine Methane Emissions" in The Australian Institute of Energy's News Journal of June 1992, and authored by the present inventor, proposed to ingest methane containing coal mine ventilation air into a gas turbines, however this was proposed prior to full realization of the problems discussed above. At the time it was proposed that a water seal be used as a flame arresting and mine isolation device between the mine and the turbine combustion system, together with a special control valve for varying an amount of dilution air to ensure the intrinsic safety of operation of the turbine should the methane in the ventilation air exceed the pre-determined upper level of methane in ventilation air. This proposed system also had the problem of being relatively complex and the turbine would, without the safety control device, operate with a methane in air content of about 3% by volume, or higher if combustion was not complete. This would be unacceptably close to the lower explosive limit of methane in air.
A yet further problem exists, in that if a mixture of fuel gas in air is to be provided for ingestion in a gas turbine or supplied to a combustor, and the mixture is to be below the lower explosive limit, explosive mixtures will exist transiently at the point of mixing. For gas turbines and large industrial units, low pressure drops in the mixing stage are desirable, and the risk and magnitude of any possible explosion should be minimal despite the large volumes of air and fuel gas required for gas turbines and industrial furnaces.
Despite the benefits of utilizing the otherwise wasted gas, until now turbine manufacturers have not proposed a suitable turbine design.
More generally, Nitric Oxide (NOX) formation during the combustion of fuel gases is a general problem for a wide range of heating systems such as commercial and industrial combustors, furnaces, gas turbines and the like. In many applications, the desired temperature of the combustion gases formed by the combustion process is significantly less than the flame temperature of the gas burnt, however the level of NOX produced in the resultant gas is determined by the flame temperature and not the temperature of the final mixed gases. Typical of this problem is the combustion of fuel in gas turbines where the combustion gases are required at temperatures in the order of 850.degree. C. to 1,200.degree. C., but where NOX levels are determined by flame conditions in the combustor, even with very advanced designs for burners (such as rich burn followed by lean burn) which are expected to reduce NOX in exhaust gases to below 20 ppm, the possibility of reducing NOX emissions to below 10 ppm is generally thought to be achievable only with the use of very special combustion systems such as catalytic devices.
Catalytic combustion systems generally rely on the use of noble metal catalysts such as platinum and palladium which have the problem of being rare and expensive elements, and which are also easily poisoned by impurities in some fuel gases, one example being the presence of silanes in landfill gases.
A further combustion problem exists where mixtures of gaseous fuel and air exist and the level of fuel in the air is below the lower flammability of the mixture, hence cannot be combusted using currently available non-catalytic combustion technology without requiring the combustion of additional fuel to initiate and maintain combustion.
It is known to ingest hydrocarbon contaminated air in gas turbins to combust the contaminant and utilize it as part of the turbine fuel, but generally only where the content represents a minor portion of the fuel. This technique is used primarily to reduce the emissions of the contaminant, and cannot be readily used to supply a significant portion of the gas turbine's fuel requirements using conventional gas turbines due to limitations of conventional combustors.
It is a preferred objective of the present invention to overcome or at least ameliorate one or more of the foregoing problems in the prior art.